In the current Ordained Servant, edited by Gregory Reynolds there’s a succinct treatment of natural law by Dr. David Van Drunen. Van Drunen rightly points out that natural law, despite howls of protest over its novelty, is well-established in Reformed confessions:
one cannot extract natural law from the system of doctrine taught in the standards without fundamentally damaging the system itself. Natural law is integral to the historic Reformed system of doctrine. In light of all this, I believe that we who are confessional Presbyterians do not have an option about whether to affirm a robust doctrine of natural law as part of our system of doctrine. Our challenge is to develop a theology of natural law from Scripture that best illuminates and further refines this confessional material.
If you want more detail, we have previously considered specific places where the Westminster Confession of Faith incorporates natural law.
Maybe you have wondered why, if non-Christians are as bereft of proper presuppositions as worldviewists portray them to be, so many of them seem quite prudent and moral. Van Drunen shows how natural law provides an explanation:
. . . natural law is a tool of common grace for the preservation of human society. This corresponds to what is often termed the second use of the law. The story of Abraham and Abimelech in Genesis 20 provides a good illustration. Sojourning in Gerar, Abraham deceived king Abimelech by calling Sarah his sister, and Abimelech promptly took her into his home. Informed of the real situation by God in a dream, Abimelech confronted Abraham the next morning. Though they came from different places, cultures, and religions, Abimelech accused him: “You have done to me things that ought not to be done” (20:9). This pagan recognized a universal standard of morality, cutting across cultural and ethnic divides, that one person should be able to expect any other person to acknowledge. Abraham’s response—“I did it because I thought, There is no fear of God at all in this place, and they will kill me because of my wife” (20:11)—displays that he had misjudged Gerar. There was indeed a certain (non-redemptive) fear of God in this place that restrained the outbreak of sin. The natural law is an instrument of common grace.
Then Van Drunen explains a connection between natural law and moral reasoning that does justice to sorting out situations that aren’t easily susceptible to proof-texts:
natural law shows believers how we are to live well in a dangerous world. Scripture makes clear that the moral life is not just about memorizing rules, but also about observing the world, learning how things work, and drawing appropriate moral conclusions. The wisdom commended in Proverbs is inconceivable without natural law. The structure of the universe is suffused with God’s wisdom, by which he made it (Prov. 8:22–31), and by perceiving and following this wisdom human beings find success and blessing in the world (8:15–21, 32–36). Observation of the world should lead believers to conclusions about how it regularly operates, and this in turn should compel certain moral conclusions.
There’s more, like Van Drunen’s grounding of natural law in the Noahic covenant. If you haven’t considered the topic yet, the article is a solid introduction to natural law from a Reformed perspective.